Infrared, raman, and 13C NMR spectra of two crystalline forms of (1R,3S)-3-(p-thioanisoyl)-1,2,2-trimethylcyclopentanecarboxylic acid

Assessment of the solid-state composition of an active salicylanilide compound by FT-Raman spectroscopy

A biologically active salicylanilide compound currently appears in three known solid-state forms: polymorph I (Pol I), polymorph II (Pol II) and the amorphous form (Amorph). The obtained FT-Raman spectra revealed several regions of interest (ROIs) qualitatively distinguishing the different forms, allowing samples with an unknown polymorphic composition to be quantitatively analysed by FT-Raman spectroscopy. The Markov-transformed peak areas of the Raman-bands in the ROIs from the samples were determined and compared with the transformed peak areas obtained for the reference solid-state forms. A constrainted linear regression model estimated the contribution of each reference to the different samples. The applicability of this approach was demonstrated by analysing commercially available batches.

NMR studies of organic polymorphs and solvates

This review article describes the applications of NMR to the study of polymorphs and related forms (solvates) of organic (especially pharmaceutical) compounds, for which it is of increasing academic and practical importance. The nature of the systems covered is briefly introduced, as are the techniques constituting solid-state NMR. The methodologies involved are then reviewed under a number of different headings, ranging from spectral editing through relaxation times to shielding tensors and NMR crystallography. In each case the relevant applications are described. Whilst most studies concentrate on structural matters, motional effects are not neglected. A special section discusses studies of solvates (especially hydrates), and another reviews quantitative analysis.

Structural and analytical characterization of three hydrates and an anhydrate form of risedronate

Four hydration states are reported for Risedronate monosodium. A single-crystal X-ray structure determination is provided as proof of assignment for the monohydrate, hemi-pentahydrate, and variable hydrate forms. The structure provided for the anhydrate form was determined through simulating annealing calculations and subsequent Reitveld refinement of a high-quality X-ray powder diffraction patterns Favorable comparisons of experimentally obtained X-ray powder patterns are made to those generated from the single crystal data. Characteristic infrared, Raman, and NMR spectra are provided and discussed for each form as are thermal analysis profiles. In addition, photomicrographs are provided for each of the forms isolated for this study. The hemi-pentahydrate is demonstrated to be the equilibrium form at room temperature and 37 degrees C, in the presence of water.

Spectroscopic characterization of interactions between PVP and indomethacin in amorphous molecular dispersions

PURPOSE

To study the molecular structure of indomethacin-PVP amorphous solid dispersions and identify any specific interactions between the components using vibrational spectroscopy.

METHODS

Solid dispersions of PVP and indomethacin were prepared using a solvent evaporation technique and IR and FT-Raman spectra were obtained.

RESULTS

A comparison of the carbonyl stretching region of gamma indomethacin, known to form carboxylic acid dimers, with that of amorphous indomethacin indicated that the amorphous phase exists predominantly as dimers. The hydrogen bonding of alpha indomethacin is not as dimers. Addition of PVP to amorphous indomethacin increased the intensity of the infrared band assigned to non-hydrogen bonded carbonyl. Concomitantly, the PVP carbonyl stretch appeared at a lower wavenumber indicating hydrogen bonding. Model solvent systems aided spectral interpretation. The magnitude of the spectral changes were comparable for an indomethacin-PVP solid dispersion and a solution of indomethacin in methylpyrrolidone at the same weight percent.

CONCLUSIONS

Indomethacin interacts with PVP in solid dispersions through hydrogen bonds formed between the drug hydroxyl and polymer carbonyl resulting in disruption of indomethacin dimers. PVP may influence the crystallisation kinetics by preventing the self association of indomethacin molecules. The similarity of results for solid dispersions and solutions emphasises the "solution" nature of this binary amorphous state.

Quantitative FT-Raman analysis of two crystal forms of a pharmaceutical compound

A pharmaceutical active compound H appears in two polymorphs, A and B, that are stable below and above room temperature, respectively. The A- and B-forms were found to have distinct FT-Raman spectra, in particular for a band at 1716 cm-1 (A-form) or 1724 cm-1 (B-form). Mixtures of A- in B-form were prepared, and the relative intensity of the characteristic bands at 1716 and 1724 cm-1 was found to be proportional to the relative amounts of A- and B-form in the mixtures. A calibration was made which was linear in the range from 1.8 to 15.4% (w/w) of A- in B-form. The results were compared with other methods for analysis of polymorphs: FT-IR spectrometry, differential scanning calorimetry, and powder X-ray diffractometry. A novel FT-Raman sample presentation method for inhomogeneous samples is presented.

Inferring thermodynamic stability relationship of polymorphs from melting data

This study investigates the possibility of inferring the thermodynamic stability relationship of polymorphs from their melting data. Thermodynamic formulas are derived for calculating the Gibbs free energy difference (delta G) between two polymorphs and its temperature slope from mainly the temperatures and heats of melting. This information is then used to estimate delta G, thus relative stability, at other temperatures by extrapolation. Both linear and nonlinear extrapolations are considered. Extrapolating delta G to zero gives an estimation of the transition (or virtual transition) temperature, from which the presence of monotropy or enantiotropy is inferred. This procedure is analogous to the use of solubility data measured near the ambient temperature to estimate a transition point at higher temperature. For several systems examined, the two methods are in good agreement. The qualitative rule introduced this way for inferring the presence of monotropy or enantiotropy is approximately the same as The Heat of Fusion Rule introduced previously on a statistical mechanical basis. This method is applied to 96 pairs of polymorphs from the literature. In most cases, the result agrees with the previous determination. The deviation of the calculated transition temperatures from their previous values (n = 18) is 2% on average and 7% at maximum.